Steric Effects in the Reaction of Electrogenerated Ligand-Reduced Nickel Salen with Organic Halides

Abstract

Cyclic voltammetry (CV) and controlled-potential electrolysis were employed to examine the reactions of electrogenerated ligand-reduced nickel salen with four different organic halides (1-bromooctane, benzyl bromide, 1-bromomethylnaphthalene, and a-bromodiphenylmethane). CVs for nickel(II) salen in the presence of benzyl bromide or 1-bromomethylnaphthalene exhibit characteristic features for the catalytic reduction of substrates involving radical intermediates. Bulk electrolyses of benzyl bromide and 1-bromomethylnaphthalene at carbon cathodes catalyzed by nickel salen were also carried out at selected potentials to afford various products. Comparison of the CVs for nickel salen before and after reactions with these four organic halides further reveals that steric effect could play an important role in the corresponding nucleophilic attack of the substrates by ligand-reduced catalyst (a radical-anion), which follows the sequence of 1-bromooctane > benzyl bromide > 1-bromomethylnaphthalene > α-bromodiphenylmethane in terms of reaction efficiency. A proposed mechanism for the electrochemical reactions was also discussed on the basis of previous studies.